The present invention relates to a prosthetic acetabular cup inserter and impactor, for use particularly, but not exclusively, in minimal invasive surgery (MIS) with a short incision.
Prosthetic acetabular cup inserter and impactors are used to implant prosthetic acetabular cups into the cavity of a patient's hip, and generally comprise an elongate straight or curved body with a cup engaging head at a first end thereof, and a handle and impaction anvil at a second end thereof. The surgeon releasably fits a cup implant to the cup engaging head, and then positions the cup inside the patient's hip. He then applies a hammering force to the impaction anvil to secure the cup in place, before releasing the cup from the cup engaging head.
There are several known types of cup engaging head, which use different methods to releasably engage a cup. A common arrangement is to provide a screw threaded hole in the top of the cup, which can be fitted to a threaded head at a first end of the body. Such an arrangement is shown in U.S. Pat. No. 7,621,921 in the name of SYMMETRY MEDICAL, INC. In this case a threaded head is provided at a first end of a curved body, onto which is screwed a cup with a screw threaded hole. To release the cup from the inserter and impactor the threaded head is collapsed. This is necessary because the threaded head is not rotatable in relation to the body of the inserter and impactor.
U.S. Patent Application Publication No. 2008/0021481 in the name of BURGI discloses another screw thread arrangement, but this time the threaded head is disposed at the end of an axially rotatable drive train carried by the body of the inserter and impactor. Therefore, a handle at the end of the drive train is rotated to screw the threaded head into and out of the cup.
Other devices owned by the assignee of the present invention or its affiliates are U.S. Pat. Nos. 7,341,593, 7,462,180 and 7,993,348, and U.S. Patent Application Publication Nos. 2011/0130763 and 2011/0184423
However, these devices cannot function with cups which are not provided with screw threaded holes, which may be preferred for various reasons. Therefore, it is also known to releasably engage a cup using an outwardly expandable cup engaging element applied to the inner surface of the cup.
U.S. Patent Application Publication No. 2007/0270783 in the name of ZUMSTEG et al discloses a cup engaging head comprising an expandable metal ring which engages the inner surface of the cup by being forced radially outwardly by relative axial movement of a cone arranged inside it. The ring has a plurality of projections which engage corresponding radial grooves in the inner surface of the cup. This arrangement does away with the requirement for a screw threaded hole, but requires radial grooves instead, which again may not be desirable from both a performance and manufacturing point of view.
U.S. Patent Application Publication No. 2004/0215200 in the name of TORNIER et al discloses a cup engaging head comprising a radially deformable supple ring, which again engages the inner surface of the cup by being forced to expand by relative axial movement of a cone. However, this time the ring comprises a plurality of resilient petals which are simply pressed against the inner surface of the cup to engage it. This is advantageous because the inner surface of the cup can be free from any engagement formations.
In U.S. Pat. No. 7,341,593 and U.S. Patent Application Publication No. 2011/0130763, both in the name of the applicant, there are disclosed further prosthetic acetabular inserter and impactors which engage the inner surface of a cup by means of a resilient ring. In U.S. Pat. No. 7,341,593 a cylindrical flexible ring is disposed on a plate at the first end of the body, and an axial displaceable operating member with an outer rim overlying the ring is drawn towards the plate to compress the ring so it if forced radially outwardly against the inner surface of the cup. In U.S. Patent Application Publication No. 2011/0130763 a rather different configuration is disclosed, in which an expandable resilient ring is drawn back onto a static expander cone to force it to radially outwardly expand and engage the inner surface of the cup.
However, all of these resilient expandable rings suffer from a certain problem when they are used with a particular type of cup. Prosthetic acetabular cups are commonly polished to a high degree on their inner surface, and this process creates a part-spherical inner bearing surface which is slightly larger than a hemisphere. In other words, the outer rim of the cup has a smaller diameter than the largest diameter of the inner bearing surface. In most cups there is a substantially cylindrical neck portion extending from the inner bearing surface to the outer rim. This means that when resilient radially outwardly expanding rings like those described above are used to engage a cup, the ring actually engages the neck portion, and not the inner bearing surface. This is because all known rings are either cylindrical in shape, or hemi-spherical, and therefore come into contact with the neck of the cup due to its smaller diameter. This is not an issue when the neck of the cup extends far enough for a sufficient grip to be established, and this is the case with known cups with a flat outer rim.
However, U.S. Pat. No. 7,833,276, in the name of the applicant discloses a prosthetic acetabular cup comprising an inner part-spherical bearing surface and an outer peripheral rim portion which is shaped to mimic the contours of the natural shape of an acetabulum. The outer peripheral rim portion has an illium rim portion, an ischium rim portion and a pubis rim portion. The portions of the rim between the pubis rim portion and the illium rim portion and between the pubis rim portion and the ischium rim portion are concave and slightly more proximal than the pubis rim portion; the portions of the rim between the illium rim portion and the pubis rim portion, and between the illium rim portion and the ischium rim portion are more concave and proximal than the illium rim portion; and the portions of the rim between the ischium rim portion and the illium rim portion and between the ischium rim portion and the pubis rim portion are more concave and proximal than the ischium rim portion.
With this construction of cup, the neck portion is particularly short at the concave portions between the illium rim portion, the ischium rim portion and the pubis rim portion, and at the pubis rim portion itself. In fact, the neck only really has any significant extent at the illium rim portion and the ischium rim portion. The result is that when the known resilient radially outwardly expanding rings are used with these cups, engagement is only really achieved at the illium rim portion and the ischium rim portion, and this is not sufficient to properly engage the cup for insertion into the patient.
The present invention is intended to overcome some of the above problems.